What is a sex-linked trait and why are males more frequently affected?

• A. A trait carried on the X chromosome; males have only one X, so a single allele is expressed
• B. A trait on the Y chromosome; males have only one Y
• C. A trait that is autosomal and equally expressed in both sexes
• D. A trait inherited from mitochondria causing maternal inheritance

Answer: (A)

Sex-linked traits are genes located on the X chromosome. In humans, the X carries many important genes, and males have only one X (and a Y). Because there isn’t a second X to mask it, a single allele on the male’s X is expressed in the phenotype. That means if a recessive allele for a trait exists on his single X, he will show the trait, whereas a female would need two copies of that recessive allele to express it. This is why such traits commonly appear more in males.

The other options don’t fit the usual idea of a sex-linked trait: a Y-linked trait would affect males, but most sex-linked traits we discuss are on the X; an autosomal trait would affect both sexes roughly equally; and a mitochondrial (maternal) inheritance pattern is separate from sex chromosomes.